David Cameron has been warned that he will have to endorse sweeping changes to the government's planned NHS reforms when a senior adviser to Nick Clegg threatened to resign unless a series of demands are met.

Norman Lamb, a government whip who is the Liberal Democrat leader's senior parliamentary adviser, said his party's MPs and peers would be unable to support the health and social care bill if their concerns are ignored.

Lamb's warning came as the British Medical Association claimed the tight NHS settlement, which will raise its budget in line with inflation, is leading to an "accelerating withdrawal of services". Growing numbers of patients are being denied treatment for conditions such as infertility.

Lib Dem sources said Lamb, who briefed Clegg in advance of his intervention on the BBC's Politics Show, was not speaking on behalf of the deputy prime minister. Clegg is making his own suggestions within government during a two-month "listening exercise" on the NHS reforms launched last week. But the Guardian understands Lamb's four demands will need to be met if Lib Dem MPs and peers are to vote in favour of the bill when it is revived in June after the "listening exercise".

Lamb, the former Lib Dem health spokesman, demanded changes to the bill that would abolish primary care trusts and hand 60% of the NHS budget to GP-led consortiums by 2013. They include:

• Abandoning the 2013 deadline and adopting an "evolution, not revolution" approach.

• Following the school reforms and allowing GPs to opt into consortiums.

• Keeping "clusters" of PCTs to observe the "performance management" of GPs.

Lamb told the BBC that he accepted reform of the NHS was vital and agreed with handing greater powers to GPs. But he warned that rushed reforms would pose a "financial risk" to the NHS.

"My real concern is the financial risk of doing it too quickly, because then you lose services, patient care suffers," he said. "The financial risk is that at the moment the plan is to transfer responsibility to GP consortiums, new organisations. There's no evidence about how these organisations will work but they're supposed to be up and running by April 2013."

Lamb indicated that unless his demands were met the bill would be blocked by Lib Dems. "Let's stick to the principle, which is really good, but let's not destroy it by getting the process wrong," he said. "This package is not going to work unless we can get people on board and that includes addressing the concerns that I've raised." He made it clear that he would resign if he is not happy: "I've said that if it's impossible for me to carry on in my position I will step down. I don't want to cause embarrassment but I feel very strongly about this issue."

Andrew Lansley, the health secretary who fell out with Lamb before the election during a row over the imposition of a so-called "death tax", vetoed his appointment as a minister. Lansley will struggle to meet Lamb's demands because he is likely to feel that allowing GPs to opt into the reforms, in the way that schools can choose to become academies, would risk creating a two-tier health service.

Lansley will embark on the latest stage of his listening exercise when he hears from nurses at the Royal College of Nursing annual congress. Dr Peter Carter, chief executive of the RCN, expressed support for Lamb's criticisms: "Norman Lamb is a man that knows his way round the health service. He has got a huge amount of credibility. The government would do well to be cognisant of what Norman Lamb is saying."